in chapter 1, in basement of the tavern 2nd room. There are 4 of them.

in chapter 2, Brothel(tents) outside Kaedweni camp near Blue stripes encampment.
Talk to madame and she will offer you to choose between her 5 prostitutes.

If you chose Roche's path you can access them anytime.

If you chose Ioverth's path you can access them only during the quest "Where Is Triss Merigold?" when you get sent near kaedweni camp.

Note: almost all prostitutes have unique "introduction" scene.However the sex scenes are shared between them,game randomly picks animation out of few possible one and plays it with the corresponding prostitute model.

Triss in chapter 1, on the quest The Rose of Remembrance.

Ves in chapter 2 (roche's path),on the quest "Ave Henselt!" after you defeat all opponents in the arena.

Mottle in chapter 2 (Ioverth's path), if you saved her from burning building in chapter 1 she will be in front the western entrance to the Vergen.

Cynthia in chapter 3 (Ioverth's path), on the quest "The Secrets of Loc Muinne". Available only if you have "Enhanced Edition" of the game.

There are also some independent nude cutscenes:

Triss

In the Prologue she is in bed with geralt.

In the beginning of chapter 3 (Ioverth's path) if you help Philippa lift the curse Triss will be transformed from figurine naked.

Baroness LaVallette If you killed Arayan LaVallette in prologue then during the prison escape if you go through the floor hatch in one of the first cells after you get loose you will find her topless and about to be tortured.

Phillipa in chapter 2 (Ioverth's path), in Vergen there are 2 scenes with her and her "apprentice",inside her house.

Saskia in chapter 2 (Ioverth's path), on the quest in the harpy den if you collect all crystals and watch them on projector Saskia will appear naked in a dream of some dwarf.

Spoiler alert: I don't know about romance, but when it comes to full digital nudity then if you follow Ioerveth's path there's a Succubus, the elf you saved in chapter 1 and there are always the hookers...